Multi-level transistor fabrication method with a patterned upper transistor substrate and interconnection thereto

ABSTRACT

A process is provided for producing active and passive devices on various levels of a semiconductor topography. As such, the present process can achieve device formation in three dimensions to enhance the overall density at which an integrated circuit is formed. The multi-level fabrication process not only adds to the overall circuit density but does so with emphasis placed on interconnection between devices on separate levels. Thus, high performance interconnect is introduced whereby the interconnect is made as short as possible between features within one transistor level to features within another transistor level. The interconnect employs a via routed directly between a well of an upper level transistor to a well of a lower transistor so as to effect direct coupling between the wells of the respective transistors. Direct coupling in this fashion affords consistent operation of transistors arranged on separate elevation levels. The via is made as short as possible so as to reduce any discrepancy in substrate/well voltage potential. This ensures predictable operation of transistors fashioned on separate elevation levels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to integrated circuit manufacture and more particularly to multi-level transistor fabrication and high performance interconnect arranged therebetween.

2. Description of the Relevant Art

The structure and the various components, or features, of a metal oxide semiconductor ("MOS") are generally well known. A MOS transistor typically comprises a substrate material onto which a patterned gate conductor is formed. The gate conductor serves to self-align impurities forwarded into the substrate on opposite sides of the gate conductor. The impurities placed into the substrate define a junction region, also known as source/drain regions. The gate conductor is patterned from a layer of polysilicon using various lithography techniques.

A typical n-channel MOS transistor employs n-type junctions placed into a p-type substrate. Conversely, a typical p-channel MOS transistor comprises p-type junctions placed into an n-type substrate. The substrate can extend entirely across a monolithic silicon wafer or can be classified as a "well" which extends only across a portion of the wafer. Thus, the term substrate is used liberally henceforth and is defined to include either the entire wafer area or only a portion of the wafer, to interchangeably encompass the term well. The substrate (or well) is doped with an impurity type opposite the junction areas formed therein.

A pervasive trend in modern integrated circuit manufacture is to produce transistors having feature sizes as small as possible. To achieve a high density integrated circuit, features such as the gate conductor, source/drain junctions, and interconnect to the junctions must be as small as possible. Many modern day processes employ features which have less than 1.0 μm critical dimension. As feature size decreases, the resulting transistor as well as the interconnect between transistors also decreases. Smaller transistors allows more transistors to be placed on a single monolithic substrate, thereby allowing relatively large circuit systems to be incorporated on a single, relatively small die area.

The benefits of high density circuits can only be realized if advanced processing techniques are used. For example, semiconductor process engineers and researchers often study the benefits of electron beam lithography and x-ray lithography to achieve the lower resolutions needed for submicron features. To some extent wet etch has given way to a more advanced anisotropic (dry etch) technique. Further, silicides and polycides have replaced higher resistivity contact structures mostly due to the lower resistivity needed when a smaller contact area is encountered.

There are many numerous other techniques used to achieve a higher density circuit, however, these techniques as well as others still must contend with problems resulting from higher density itself. Even the most advanced processing techniques cannot in all instances offset the problems associated with small features or features arranged extremely close to one another. For example, as the channel length decreases, short channel effects ("SCE") generally occur. SCE cause threshold voltage skews at the channel edges as well as excessive subthreshold currents (e.g., punch through and drain-induced barrier lowering). Related to SCE is the problem of hot carrier injection ("HCI"). As the channel shortens and the supply voltage remains constant, the electric field across the drain-to-channel junction becomes excessive. Excessive electric field can give rise to so called hot carriers and the injection of those carriers into the gate oxide which resides between the substrate (or well) and the overlying gate conductor. Injection of hot carriers should be avoided since those carriers can become trapped and skew the turn-on voltage of the ensuing transistor.

It appears as though even the most advanced processing techniques cannot avoid in all instances the problems which arise as a result of high density fabrication. As features are shrunk and are drawn closer together across a single topological surface, the closeness of those features causes numerous problems even under the most advanced processing conditions. It therefore appears that there may be a certain limitation beyond which feature sizes cannot be reduced if those features are to reside on the single elevational level. It would therefor be desirable to derive a processing technique which can produce features on more than one level. That is, it would be beneficial that this multi-level processing technique produce both active (transistors) and passive (capacitors and resistors) in three dimensions so as to enhance the overall circuit density without incurring harmful side effects associated with feature shrinkage and closeness.

In order to form transistors on separate elevation levels, careful attention must be paid to the interconnection between transistors on those levels. Therefore, it is desirable to derive an interconnect scheme which can connect one or more features on one elevation (topological) level to one or more features on another level. That interconnection must be made as short as possible in order to avoid voltage discrepancies between the features being connected. Of particular importance is the need to minimize voltage differential as seen by the channel region of a lower level transistor and the channel region of an upper level transistor. It is therefore imperative that a low resistance contact be made between transistor channels on separate elevation levels. A process which forms a multi-level transistor must therefore produce, during the process sequence, low resistance interconnect between channel regions arranged on separate elevation levels for the purpose of maintaining consistence operation among those transistors. If resistance between wells is too high, then one transistor may turn on or off prematurely relative to the other--an outcome which should be avoided if high performance circuit operation is desired.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by a multi-level transistor fabrication technique. The present technique can produce one or more active and passive devices on a first level, followed by one or more active and passive devices on a second level. The first level is substantially planar and extends across an entire wafer surface. The second level is also substantially planar and parallel to the first level, but spaced by a dielectric therefrom.

An active device formed on the first level is termed a first level transistor, or more simply a first transistor. Similarly, an active device formed on the second level is termed a second level transistor, or second transistor. The first transistor is formed within and upon a first level substrate, or first substrate. A second level substrate (or second substrate) accommodates the second transistor and is spaced above the first level substrate by an interlevel dielectric. According to one embodiment, the first level substrate comprises a similarly doped implant area. This implant area is termed a well-connect implant. Being doped the same as but with higher concentration than the first level substrate, the well-connect implant is electrically connected with the first level substrate. The second level substrate comprises a region of polysilicon material into which and upon which the second transistor is formed. The second level substrate is doped with an impurity of the same type as that used in the first level substrate and the well-connect implant. The well-connect implant is spaced a lateral distance from the first transistor within the first level substrate. The first level substrate which encompasses the first transistor junctions and the well-connect implant may interchangeably be called a well. It is recognized that the second level substrate can also be interchangeably called a well. Regardless of the nomenclature use, a substrate (or well) defined herein is one having a well-connect implant at the first topological level and a polysilicon-based material at the second topological level.

The second transistor is spaced directly above the well-connect implant. Specifically, the second level substrate of the second transistor is spaced directly above the well-connect implant. A via extends along a vertical axis directly between the second level substrate and the well-connect implant. Accordingly, the via is conductive and made as short as possible so as to minimize voltage discrepancy between the first and second level substrates (i.e., between the first and second transistor channels).

As a result of the direct path taken by the conductive via, low resistance and therefore minimum voltage disparity exits between upper and lower substrate/well regions. Direct via routing is achieved by aligning the second transistor a spaced distance above yet laterally offset from the first transistor. The conductive via is of predefined width and extends from the bottom surface of the second transistor substrate along an axis defined as perpendicular to the planes in which the first and second level transistors are placed. That axis is preferably vertical and relatively short as defined by the thickness of the interlevel dielectric which separates those planes.

Broadly speaking, the present invention contemplates a method for interconnecting a well region of a first transistor to a well region of a second transistor. The well region of the first transistor is defined within the first level substrate, and the well region of the second transistor is defined within the second level substrate. A process is therefore presented for forming connection between the first transistor substrate and the second transistor substrate. Thus, the first transistor substrate encompasses a well-connect implant, a source region and a drain region. The source and drain regions are spaced from one another about a channel region of the first transistor. A first dielectric (i.e., first interlevel dielectric) is deposited across the first transistor as well as the topography lateral to the first transistor. An opening is then etched through the first dielectric to the well-connect implant. The opening is then filled with a metal, and a second level substrate is deposited entirely across the first dielectric and metal upper surfaces. The second level substrate is then removed in all regions except for the area directly above the metal and a contiguous area laterally spaced therefrom. The removed regions of second level substrate are then filled with a second dielectric. The second dielectric can, however, be formed either before or during formation of a second transistor. The second transistor is formed by forwarding dopants and depositing films into and upon the retained second level substrate to render the second substrate into the second transistor.

The present invention further contemplates an integrated circuit. The integrated circuit comprises an interconnect routed between the well-connect implant of the first transistor and the second transistor substrate. The interconnect comprises a conductive via extending along a vertical axis between opposed upper and lower via portions. A lowermost surface of the lower portion is configured to abut against the well-connect implant arranged within a first substrate. An uppermost surface of the upper portion is configured to abut against a second substrate which resides a dielectric distance above the first substrate. A second transistor is configured entirely within the second substrate.

Preferably, the first substrate into which the well-connect implant and source/drain implants of the first transistor are formed comprises a silicon-based material. The silicon-based material is coated with an oxide of relatively uniform thickness. The oxide thereby serves as a gate oxide for the first transistor. Similarly, a uniform thickness oxide is formed upon the polysilicon which forms the second substrate. That oxide upon the second substrate serves as a gate oxide for the second transistor. The metal used to fill the opening to form the interconnect via between the second transistor substrate and the first transistor substrate is either titanium, tungsten, titanium nitride or aluminum (i.e., Ti, W, TiN or Al), or a combination of these metals.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:

FIG. 1 is a partial cross-sectional view of a semiconductor substrate having a gate conductor patterned upon the substrate according to an initial processing step of the present invention;

FIG. 2 is a partial cross-sectional view of the semiconductor substrate having implant regions placed within the substrate to form a first transistor with a well-connect implant spaced from the transistor according to a processing step subsequent to FIG. 1;

FIG. 3 is a partial cross-sectional view of the semiconductor substrate having a first dielectric layer deposited across the first transistor according to a processing step subsequent to FIG. 2;

FIG. 4 is a partial cross-sectional view of the semiconductor substrate having an opening formed through the first dielectric to the well-connect implant according to a processing step subsequent to FIG. 3;

FIG. 5 is a partial cross-sectional view of the semiconductor substrate having a metal via formed within the confines of the opening according to a processing step subsequent to FIG. 4;

FIG. 6 is a partial cross-sectional view of the semiconductor substrate having a layer of substrate material blanket deposited upon the first dielectric and metal via according to a processing step subsequent to FIG. 5;

FIG. 7 is a partial cross-sectional view of the semiconductor substrate having the layer of second substrate material removed except for a region above the metal via to form a second substrate according to a processing step subsequent to FIG. 6;

FIG. 8 is a partial cross-sectional view of the semiconductor substrate having a second dielectric formed in areas removed of the layer of second substrate material according to a processing step subsequent to FIG. 7; and

FIG. 9 is a partial cross-sectional view of the semiconductor substrate having a gate conductor formed upon the second substrate and source/drain implants placed within the second substrate to complete a second transistor according to a processing step subsequent to FIG. 8.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 illustrates a partial cross-sectional view of a semiconductor substrate 10. Substrate 10 is preferably a silicon-based, single crystalline material doped either n-type or p-type. Arranged on the upper surface of substrate 10 are various isolation structures (not shown). Isolation structures can be formed either by a shallow-trench process or the well known LOCOS process. The isolation structures serve to isolate active devices placed within and upon substrate 10.

Formed across the entire substrate 10 is a dielectric 12. Dielectric 12 is often referred to as a gate dielectric. Dielectric 12 can be formed by growing an oxide upon the silicon-based substrate 10, or by depositing an oxide which, according to one embodiment, can contain nitrogen species. Deposited upon dielectric 12 is a layer of polycrystalline silicon ("polysilicon"). Select regions of polysilicon 14 and dielectric 12 are removed using conventional lithography: pattern, develop and etch cycles. The retained region is defined as an active region or, more specifically, the region in which a gate conductor 16 and gate dielectric 18 are formed. Gate conductor 16 and gate dielectric 18 are thereby the retained portions of polysilicon 14 and dielectric 12, respectively.

FIG. 2 illustrates a processing step subsequent to FIG. 1 whereby dielectric spacers 20 are formed on the opposing vertical surfaces of gate conductor 16. A lightly doped drain ("LDD") implant 22 is forwarded into substrate 10 prior to formation of spacers 20. After LDD 22 is present, and spacers 20 are produced, source/drain implants ("junctions") 24 are then formed. Prior to or after forming source/drain regions 24 is a processing step used to produce well-connect implant 26. Well-connect implant 26 is spaced from the area encompassed by first transistor 30. The amount of spacing is one such that implant 26 is electrically isolated from source/drain region 24 as well as LDD region 22. Thus, well-connect implant 26 is doped with an impurity opposite the impurity used in source/drain region 24 and LDD region 22. For example, well-connect implant 26 can comprise a p-type dopant, whereas the junction implants used to form n-channel transistor 30 can comprise n-type dopant. Conversely, well-connect implant 26 is n-type if transistor 30 is a p-channel transistor.

Formation of the source/drain region 24, LDD region 22, spacer 20, and well-connect implant 26 are processing steps which follow a particular sequence generally known to those skilled in the art. Regardless of the process sequence chosen, it is important to maintain adequate spacing between well-connect implant 26 and the outer periphery of source/drain region 24. The amount of spacing is dependent upon the implant species used and the range (depth) in which those species are forwarded into substrate 10. It is also important that the impurity type used to form well-connect implant 26 be of the same type found within substrate 10. As such, well-connect implant 26 forms a low resistance path to substrate 10. More specifically, well-connect implant 26 electrically couples to substrate 10 and specifically the channel region within substrate 10 directly beneath gate conductor 16 and gate dielectric 18.

FIG. 3, depicts a processing step subsequent to FIG. 2, wherein a first dielectric layer 32 is deposited across the transistor 30 as well as the topological surface of substrate 10 in regions outside of transistor 30. As first dielectric 32 is deposited, it conforms to some degree to the upward protruding gate conductor 16 of transistor 30 to form a peak 34. Peak 34 is preferably removed by a subsequent processing step using chemical mechanical polishing ("CMP"). CMP utilizes a slurry material and a polishing pad placed on the exposed surface of first dielectric 32, whereby the pad rotates and removes the upper surfaces commensurate with the lower surfaces. According to another technique, upper surface 34 can be removed using a sacrificial etch back. In this instance, a sacrificial material is placed on the upper surface such that the recesses or valleys are filled with that material. The material upper surface is then removed at an etch rate substantially the same as the dielectric under layer. When all of the sacrificial material is removed, the remaining dielectric surface is approximately planer in that it takes on the same contours as the planer surface of the sacrificial material. Regardless of the method chosen, the outcome remains the same: to produce a somewhat planer upper surface of first dielectric 32 in readiness for subsequent processing steps.

Referring to FIG. 4, an illustration is provided of an opening 36 formed through first dielectric 32 to well-connect implant 26. Opening 36 is produced by placing a masking layer across first dielectric 32 and then patterning a masking layer such that the region to be removed is exposed. The exposed region is then subjected to an etchant which, according to one embodiment, is a dry (anisotropic) etchant. Opening 36 is shown to extend along an axis perpendicular to the upper surface of first dielectric 32 and substrate 10 so as to form a direct path to well-connect implant 26. The width of opening 36 is defined to be no greater than the area occupied by well-connect implant 26.

As shown in FIG. 5, opening 36 is filled with a conductive material 38. Preferably, conductive material 38 is a metal or metal alloy such as Ti, W, TiN or Al. Conductive material 38 is thereafter removed from the upper surfaces of first dielectric 32 but retained within the confines of opening 36. The removed portions of conductive material 38 are shown by reference numeral 38a and the retained portions of conductive material 38 are shown by reference numeral 38b. Accordingly, reference numeral 38b denotes a via. The upper surface of via 38a is commensurate with the upper surface of first dielectric 32 using, for example, CMP. Removal of portions 38a is necessary to retain the substantially planer upper surface created during formation of first dielectric 32.

Conductive material 38 can be deposited in various ways such as, for example, sputter deposition, evaporation, etc. Regardless of the deposition method chosen, material 38 fills opening 36 so that via 38b occurs.

FIG. 6 depicts the deposition of a layer second substrate material 42 upon first dielectric 32 as well as via 38b. Second substrate material 42 is made, according to one embodiment, of a polycrystalline silicon ("polysilicon") material. Material 42 is preferably formed from chemical vapor deposition ("CVD"). The CVD process is performed such that the material is deposited to a thickness sufficient to full retain source and drain junctions which are subsequently forwarded therein. If the range (depth) at which the junctions are implanted is, for example, 0.2 μm, then it is preferred that material 42 be deposited to a depth which is approximately 0.2 μm.

FIG. 7 illustrates a processing step subsequent to FIG. 6, wherein material 42 is removed except for a region directly above via 38b and except for a region above an area laterally adjacent via 38b. Material 42 can be patterned and removed using conventional photolithography techniques. Preferably, the etchant used to remove the exposed portions of material 42 comprises, according to one embodiment, a dry (anisotropic) etchant. The etch period is designed to continue for a time sufficient to remove material 42. That time can be measured by dielectric byproduct from second dielectric 32, for example.

Once material 42 is removed, all that remains is a patterned substrate material, henceforth referred to as second substrate 44. Second substrate 44 is preferably doped with an impurity species to render it conductive. Doping substrate 44 occurs either after the processing step shown in FIG. 7 or the step shown in FIG. 8.

As shown in FIG. 8, regions removed of material 42 (i.e., regions lateral to second substrate 44) can be filled with a dielectric material 46. Dielectric material is blanket deposited across the entire topography, including the upper surfaces of substrate 44 as well as dielectric 32. Thereafter, material 46 is removed from the upper surface of substrate 44 using, for example CMP or sacrificial etch. Removal continues for a time sufficient to allow retainage of dielectric material 46 is regions laterally adjacent substrate 44. The removed areas are designated with numeral 46a, while the retained areas are designated with numeral 46b. The retained portions 46b are henceforth referred to as a second dielectric which serves to electrically isolate substrate 44 for other, laterally spaced substrates.

FIG. 9 illustrates formation of the second transistor, denoted as reference numeral 46. Second transistor 46 is produced by implanting LDD and source/drain regions 48 and 50 into second substrate 44. Implant of those regions occur by a self-align process, similar to that used to produce first transistor 30. As shown, substrate 44 is electrically connected to and abuts with via 38b. As such, electrical connection is made between substrate 44 and well-connect implant 26 by a virtue of via 38b.

The processing steps used to produce second transistor 46 need not be shown for sake of brevity. However, it is understood that those steps used to produce second transistor 46 are replicated from those used in producing first transistor 30. Regardless of the steps used, second transistor 46 comprises a well region existing between source and drain regions 50. The well region of second transistor 46 is electrically linked to via 38b. Since via 38b is conductive, and preferably metal, electrical connection is effectuated thereby to well-connect implant 26. Well-connect implant 26, being of the same impurity type as substrate 10, is electrically connected to the channel of first transistor 30. Although it is of the same impurity type as substrate 10, well-connect implant 26 contains a greater concentration of impurities than substrate 10.

For sake of clarity, first transistor 30, second transistor 46, and the various features used therein are not drawn to scale. Likewise, the topological thickness of first dielectric 32 and second dielectric 46b are not drawn to scale. It is not imperative that these features be defined or that dimensions be specified, all of which would be readily apparent to those skilled in the art given the benefits described herein. All that is necessary, however, is that second transistor 46 be confined entirely upon and within substrate 44b, and that substrate 44 be electrically connected to substrate 10. Of importance to that connection is that the relatively short distance by which the vertically directed via 38b extends. This short distance, and the highly conductive nature of the metal material used in forming the via, helps to prevent a substantial voltage differential between the well regions of transistors 30 and 46. Thus, the present process is designed to enhance consistency and conformity in the operation of transistors placed on separate elevation levels.

It will be appreciated to those skilled in the art having the benefit of this disclosure that the present process methodology is capable of producing p-type and/or n-type devices in three dimensions. According to one embodiment, transistors 46 and 30 are n-type transistors, and the channel areas of those respective transistors are p-type. Alternatively, transistors 46 and 30 can be p-type transistors, with the channel areas doped n-type. In the former instance, the well-connect implant 26 is doped p-type, and in the latter instance well-connect implant 26 is doped n-type. The relative concentrations of those dopants would be generally known to those skilled in the art.

It is be understood that the form of the invention shown and described is to be taken as exemplary, presently preferred embodiments. For sake of brevity, only one transistor is shown on each elevation level. However, it is within the spirit and scope of the present invention that numerous transistors be arranged across each elevation level so that the overall circuit extends in three dimensions and that select transistors on one elevation level can be connected to select transistors on another elevation level by a virtue of a well-connect via, such as via 38b. Various modifications and changes may be made to each and every processing step without departing from the spirit and scope of the invention provided the well-connect via concepts set forth in the present claims are retained. It is intended that the following claims therefore be interpreted to embrace all such modifications and changes, and accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A method for interconnecting a substrate of a first transistor to a substrate of a second transistor, comprising:providing a first substrate encompassing a well-connect implant, a source region and a drain region, wherein said source and drain regions are spaced about a channel region of a first transistor; depositing a first layer of dielectric across said first transistor; etching an opening through said first dielectric to said well-connect implant; filling said opening with a metal; depositing a layer of polysilicon across said first dielectric and said metal and thereafter removing portions of said layer of polysilicon to form a second substrate residing directly upon and in electrical communication with said metal; forming a second layer of dielectric upon said first dielectric in regions removed of said layer of polysilicon; depositing dopants within said second substrate to render the second substrate conductive and thereafter depositing films and subsequent dopants upon and within, respectively, said second substrate to form a second transistor.
 2. The method as recited in claim 1, wherein said source region, said drain region and said well-connect implant are spaced from each other.
 3. The method as recited in claim 1, wherein said providing comprises forwarding a first dopant into said first substrate to form said source and drain regions, and forwarding a second dopant into said first substrate to form said well-connect implant.
 4. The method as recited in claim 3, wherein said first dopant is opposite said second dopant.
 5. The method as recited in claim 1, wherein said first layer of dielectric comprises oxide.
 6. The method as recited in claim 1, wherein the second substrate extends laterally across said first dielectric a pre-defined distance from said metal.
 7. The method as recited in claim 1, wherein said forming the second layer of dielectric comprises:depositing said second layer of dielectric upon said first dielectric and said substrate to a thickness which exceeds a thickness of said layer of polysilicon; and removing said second layer of dielectric to an elevation commensurate with an upper surface of said second substrate.
 8. The method as recited in claim 1, wherein said metal is selected from the group consisting of Ti, W, TiN and Al.
 9. The method as recited in claim 1, wherein said first and second transistors are vertically spaced from one another on separate elevational levels.
 10. A method for interconnecting a pair of transistors, comprising:forming a first transistor upon and within a first substrate, and forming a well-connect implant spaced from the first transistor within the first substrate; depositing, across said first transistor and said well-connect implant, a first dielectric having a substantially planar first dielectric surface; etching an opening through said first dielectric from said first dielectric surface to said well-connect implant; filling said opening with a metal; patterning a second substrate upon and in electrical communication with said metal and underlying first transistor well; and forming a source region and a drain region of a second transistor within said second substrate to form a second transistor.
 11. The method as recited in claim 10, wherein the source and drain regions are spaced apart from each other by a channel region which resides directly above said metal.
 12. The method as recited in claim 10, further comprising implanting said second substrate with a dopant of the same type as the well-connect implant.
 13. The method as recited in claim 10, wherein said forming comprises implanting said source and drain regions with an implant type opposite that of said well-connect implant.
 14. The method as recited in claim 10, wherein said etching an opening and filling said opening with a metal comprises forming a metal via which extends along an axis perpendicular to said first dielectric surface.
 15. The method as recited in claim 14, wherein said metal via comprises an upper portion and a lower portion opposite the upper portion, said upper portion abuts against said first substrate and said lower portion abuts against said well-connect implant.
 16. The method as recited in claim 10, wherein said first and second dielectrics each comprises oxide.
 17. The method as recited in claim 10, wherein said metal is selected from the group consisting of Ti, W, TiN and Al.
 18. The method as recited in claim 10, wherein said first and second transistors are vertically spaced from one another on separate elevational levels. 